It's not a case of trying. In theory, there's little stopping a designer doing something simple, but I suspect the general customer base simply wouldn't buy it.
And as Joey says, aero almost certainly plays a part - though I'd be intrigued whether it's possible to build a truck with an effectively square front end that still has decent aero. Not just good for economy remember, but also highway refinement, higher-speed acceleration etc.
Suspect it'd be difficult, largely because a lot of things that you need in a truck - high ground clearance, lots of suspension travel, large mirrors for better visibility etc - tend to all be things you don't want from an aero perspective.
Would be interesting to see what someone like
@Nessy could come up with.